Sorry just saw you said SharePoint list, using a similar method above with the rectangle and label, you would need to count all items in the gallery and divide the required width by that and multiply by the selected amount to get the width of the rectabgle. Assuming the SharePoint list is associated to a Gallery
Label.Width / Gallery1.AllItemsCount
following this you need to work out which items have been selected to do this you would use a collection and the OnUncheck/OnCheck functions
OnCheck you would add
Collect(colSelectedToggles, ThisItem)
OnUncheck use
Remove(colSelectedToggles, ThisItem)
Finally add all the bits together and the width of the rectangle is
(Label.Width)/Gallery1.AllItemsCount) * CountRows(colSelectedToggles)